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Original question:

TPS23881: no output issue

TPS23881: TPS23881 output issue

Chen Wei Hung
Prodigy 60 points
Part Number: TPS23881
Other Parts Discussed in Thread: TPS2373-4EVM-758,

Dear Darwin Fernandez Sir:

I have received TPS2373-4EVM-758 demo board and testing. But it have turn on issue cause no output voltage at no load(input need restart again to have output 5V). I don't know what is the problem? Do you have any suggestions? Thanks.    

  • 0
    TI__Mastermind 21320 points

    Hello Chen, 

    Are you using a TPS23881 PSE to power the board?
    If so, it could be possible that the MPS requirement is not met at no load, so the PSE shuts off power. This is according to IEEE802.3 standard, so this is normal. 

    In customer application, the load typically always has some current to power MCU and other functions, so no load case is very unlikely. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 60 points

    Dear Michael P.  :

    I am sure I am using TPS23881 PSE testing with TPS2373-4EVM-758 demo board. This issue was discussed with Darwin Fernandez before.You can see attached file and understand more about the matter. Thanks.

    TPS23881_ no output issue.pdf

  • 0 in reply to Chen Wei Hung
    TI__Mastermind 21320 points

    Hey Chen, 

    The restart issue can be MPS, which would pass the PoE handshake and in-rush, but fail to give the PSE the required MPS signal. 

    If the PSE cuts off power after 500 ms or so, this is due to the MPS signal not being met. The IEEE802.3 standard has a minimal Maintain Power Signature (MPS) signal that needs to be sent to the PSE at about 10 mA. Many times, during the first power up of the board, there is no load attached to the output. Some designs do not draw this much current when there is no load attached. This is more common in diode flybacks since they can more easily slip into discontinuous conduction mode (DCM), which significantly drops the overall input current. Try adding some load to the output of the design to get the input current beyond the MPS signal minimum. This behavior is characterized by a successful PoE handshake and power-up, but after 500 ms or so the input power is cut off and the output drops as well. This behavior is usually the MPS signal.

    You can see this behavior on the VDD_VSS waveform. You will see a valid PoE hanshake and startup, but after the specified time VDD_VSS goes to 0V and the cycle starts over. 

    Can you rule this out as a cause?

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 60 points

    Dear Sir:

    We really can't find any issue reasons.Can we send bad units to you for analysis? Thanks.

  • 0 in reply to Chen Wei Hung
    TI__Mastermind 21320 points

    Hello, 

    Can you send a waveform of VDD_VSS first?

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0 in reply to Michael Pahl
    Prodigy 60 points

    Dear Sir:

    Picture1:Bad unit stable waveform(CH1_VPWR; CH2_VDD)

    Picture2:Bad unit startup waveform(CH1_VPWR; CH2_VDD)

  • 0 in reply to Chen Wei Hung
    TI__Mastermind 41790 points

    Hi Chen,

    Looking at our conversation, i suspected an inrush issue with the PD you're testing. Did the EVM pass inrush? Can you take the same waveform with negotiation? If the EVM is passing and able to power up, then we can say the other PD you were testing with was marginal. And you should probably test with the EVM.